1. Field of the Invention
The present invention relates to an X-ray apparatus, and more particular to an X-ray apparatus with improved heat radiation characteristics relating to heat that is produced by, e.g. a rotation-anode type X-ray tube.
2. Description of the Related Art
An X-ray apparatus is configured to include a rotation-anode type X-ray tube in which a vacuum envelope accommodates an anode target that is rotatably supported, and a housing which accommodates the rotation-anode type X-ray tube. In a case where heat that is produced by, e.g. the anode target is to be radiated, the rotation-anode type X-ray tube is provided with a cooling mechanism for cooling the heat.
As regards X-ray apparatuses with cooling mechanisms, the following proposals have been made.
(1) An X-ray apparatus has been proposed, wherein a rotation-anode type X-ray tube and a stator are immersed in an insulating oil. A water-based coolant with a high heat transfer efficiency is made to flow through flow paths, which are partly provided at parts with high heat production, such as a recoil electron trap and a vacuum envelope provided near an anode target. Thereby, the parts with high heat production are cooled. The coolant is circulated between these flow paths and a cooling unit (see, e.g. U.S. Pat. No. 6,519,317).
(2) An X-ray apparatus has been proposed, which is constructed similarly to the X-ray apparatus (1), except that a rotation-anode type X-ray tube and a stator are immersed not in an insulating oil, but in a water-based coolant, and the water-based coolant is circulated between a housing and a cooling unit (see, e.g. PCT National Publication No. 2001-502473).
According to the X-ray apparatus with the structure (1), if the thermal load on the rotation-anode type X-ray tube increases, the heat that is produced from the outer surface of the vacuum envelope increases. However, since the coolant that cools the outer surface is only the insulating oil that is not cooled by the external exchanger. In some cases, the necessary cooling performance cannot be obtained. In addition, since the coolant contains water, metallic parts of the circulation paths may be corroded. The metallic parts, which constitute the flow paths that are partly provided at the recoil electron trap and vacuum envelope provided near the anode target, have functions to isolate the vacuum and the coolant. If corrosion progresses, such functions would deteriorate and the X-ray tube would become non-usable. If such a drawback occurs, the water-based coolant may enter the X-ray tube when the temperature of the anode target of the X-ray tube rises to a high level. The water-based coolant comes in contact with the high-temperature anode target, evaporates and raises pressure. This poses a problem in safety.
With the progress of corrosion, a suspended solid of a metal hydroxide, which is not dissolved in the coolant, may be produced. Consequently, the flow path of the coolant may be clogged by the suspended solid, and thermal transfer may be hindered or the flow rate may decrease. As a result, the cooling performance by the coolant may deteriorate. Furthermore, air, which is dissolved in the water-based coolant, becomes air bubbles with the rising of temperature of the water-based coolant and mixes into the water-based coolant. Thus, the cooling performance by the coolant may lower.
In addition to the problem of the structure (1), the X-ray apparatus with the structure (2) has the following problem. That is, with the decrease in insulation resistance due to the metal corrosion, the insulation performance of a low-voltage electric circuit system, such as a stator circuit, and the insulation performance between the housing and vacuum envelope may deteriorate. In particular, in the case where a dynamic-pressure slide bearing is used as the bearing of the rotational support mechanism, compared to the case where a ball bearing is used, the heat production of the stator increases and the electric insulation performance considerably deteriorates. In addition, the vacuum wall of the X-ray tube, which is not immersed in the water-based coolant in the case of (1), is corroded. As a result, a similar problem with the structure (1) tends to occur more easily.
Air, which is dissolved in the water-based coolant, becomes air bubbles with the rising of temperature of the water-based coolant and mixes into the water-based coolant. Thus, a similar problem with the structure (1) may occur. In addition, if such bubbles pass by an X-ray output window, the transmittance of produced X-rays may vary. If such a phenomenon occurs during use of the X-ray apparatus, X-ray images may disadvantageously be affected.
Besides, a return path of the water-based coolant communicates with the inner space of the housing, and thus low-voltage electric circuit systems are immersed in the water-based coolant. Such low-voltage electric circuit systems include a stator circuit system for supplying voltage to the stator and a turn-on getter circuit. Those parts of the stator circuit system, which are immersed in the water-based coolant, are a stator coil, wiring lines, and a current supply terminal for connection to an external power supply that is provided outside the housing. Those parts of the turn-on getter circuit system, which are immersed in the water-based coolant, are a current supply terminal for supplying current to the turn-on getter within the X-ray tube, wiring lines, and a current supply terminal for connection to an external power supply that is provided outside the housing.
Since the distances between current-conductive parts of these components are short, a problem of electric leak will arise due to a slight increase in electrical conduction (conductivity) of the water-based coolant. It is thus preferable to protect these structural components from the water-based coolant by integrally molding the components with resin. However, if a defect occurs in the mold due to long-time use, the water-based coolant flows into the structural parts within the mold, leading to electric leak.
Furthermore, both the housing and the vacuum envelope of the X-ray tube are set at ground potential. In order to prevent electrical noise in case of electric discharge of the X-ray tube, the X-ray tube is accommodated so as to be electrically insulated from the housing. Thus, in the case of the structure (2) wherein the return path of the water-based coolant communicates with the inner space of the housing, the water-based coolant is present near the insulating part between the housing and the X-ray tube. Since the distance for insulation is short, a problem of electric leak will arise due to a slight increase in electrical conductivity of the water-based coolant